The present invention relates generally to the field of lithographic patterning methods and semiconductor fabrication. More specifically this invention relates to the use of inorganic materials ligated with peroxo and organic functional groups in lithographic photoresist processes. This invention also relates to the lithographic patterning of metal oxides.
Implementation of extreme ultraviolet (EUV) lithography necessitates the development of compatible photoresists that can perform at spatial resolutions below 16 nm. Currently, traditional chemically amplified (CA) photoresists are struggling to meet the specifications for resolution, photospeed and feature roughness (termed line edge roughness, or LER) for next generation devices (Anderson, C. N.; Baclea-An, L.-M.; Denham, P.; George, S.; Goldberg, K. A.; Jones, M. S.; Smith, S. S.; Wallow, T. I.; Montgomery, M. W.; Naulleau, P., Proc. SPIE 7969, 79690R (2011)). The intrinsic image blur due to the acid catalyzed reactions that occur in these polymeric photoresists limits the resolution at small feature sizes, a fact that has been known for many years from electron-beam (e-beam) lithography. While CA photoresists are designed for high sensitivity they can suffer under EUV exposure in part because their typical elemental makeup (mainly C, with smaller quantities of O, F, S) make the photoresists too transparent at a wavelength of 13.5 nm, consequently reducing sensitivity. CA photoresists also suffer from roughness issues at small feature sizes, and experiments have indicated that the LER increases as the photospeed decreases, due in part to the nature of acid catalyzed processes. Due to the failings and problems of CA photoresists there is a need in the semiconductor industry for new types of high performance photoresists. Inorganic photoresists are one such candidate.
Previously, inorganic photoresists based on peroxopolyacids of tungsten and tungsten mixed with niobium, titanium, and/or tantalum have been reported for use as radiation sensitive materials for patterning (Kudo et. al., U.S. Pat. No. 5,061,599, 1991; H. Okamoto, T. Iwayanagi, K. Mochiji, H. Umezaki, T. Kudo, Applied Physics Letters, 49 (5), 298-300, 1986). These materials were effective in patterning large features in a bilayer configuration with deep UV, x-ray, and e-beam sources. More recently, impressive performance was shown using a cationic hafnium metal oxide sulfate (HfSOx) material with a peroxo complexing agent to image 15-nm half-pitch (HP) by projection EUV exposure (Keszler et al., US20110045406, 2011 and J. K. Stowers, A. Telecky, M. Kocsis, B. L. Clark, D. A. Keszler, A. Grenville, C. N. Anderson, P. P. Naulleau, Proc. SPIE, 7969, 796915, 2011). This system demonstrated the best performance of a non-CA photoresist and has a photospeed approaching the requirements for a viable EUV photoresist.
The hafnium metal oxide sulfate materials with a peroxo complexing agent have several practical drawbacks. First, the materials are cast out of highly corrosive sulfuric acid/hydrogen peroxide mixtures and they show significant issues with shelf-life stability. Second, these are poorly characterized, complex mixtures with no clear pathway to modify their structure to improve performance. Third, they must be developed in extremely high concentration tetramethylammonium hydroxide (TMAH) solutions of up to 25 weight percent (wt %). This represents a significant toxicological concern. Finally, though still under development, it is not clear that either of these classes of materials will ever be able to fully satisfy the performance requirements for EUV in such areas as sensitivity and line edge roughness.
Thus, inorganic compounds are needed having more favorable properties for EUV lithography, including but not limited to solubility in a suitable casting solvent, solubility in a suitable developer, high sensitivity to EUV, and compatibility with thermal treatments following film formation and/or exposure.